Here's the passage from his book, Dreams from My Father, where he discusses his drug use:
"I had learned not to care. I blew a few smoke rings, remembering those years. Pot had helped, and booze; maybe a little blow when you could afford it. Not smack, though—Mickey, my potentional intiator had been just a little too eager for me to go through with that. Said he could do it blindfolded, but he was shaking like a faulty engine when he said it. Maybe he was just cold; we were standing in a meat freezer in the back of the deli where he worked, and it couldn't have been more than twenty degrees in there. But he didn't look like he was sweating, his face shiny and tight. He had pulled out the needle and the tubing, and I'd looked at him standing there, surrounded by big slabs of salami and roast beef, and right then an image popped into my head of an air bubble, shiny and round like a pearl, rolling quietly through a vein and stopping my heart... Junkie. Pothead. That's where I'd be headed: the final, fatal role of the young would-be black man. Except the highs hadn't been about that, me trying to prove what a down brother I was. Not by then anyway. I got just the opposite effect, something that could push questions of who I was out of my mind, something that could flatten out the landscape of my heart, blur the edges of my memory. I had discovered that it didn't make any difference whether you smoked reefer in the white classmmate's sparkling new van, or in the dorm room of some brother you'd met at the gym, or on the beach with a couple of Hawaiian kids who had dropped out of school and now spent most of their time looking for an excuse to brawl.

Nobody asked you whether your father was a fat-cat executive who cheated on his wife or some laid-off joe who slapped you around whenever he bothered to come home. You might just be bored, or alone. Everybody was welcome into the club of dissaffection. And if the high didn't solve whatever it was that was getting you down, it could at least help you laugh at the world's ongoing folly and see through all the hypocrisy and bullshit and cheap moralism.

Thats how it had seemed to me then anyway. It had taken a couple of years before I saw how fates were beginning to play themselves out, the difference that color and money made after all, in who survived, how soft or hard the landing when finally fell. Of course, either way, you need some luck. That's what Pablo lacked, mostly, not having his driver's license that day, a cop with nothing better to do than check the trunk of his car. Or Bruce, not finding his way back from too many bad acid trips and winding up in a funny farm. Or Duke not walking away from the car wreck..."

In high school, Obama, according to his yearbook photo, was part of the "Choom Gang." Chooming was Hawaiian slang for smoking pot. (source)